Micrometers of the type employed by machinists, quality control persons and the like ordinarily include a measuring spindle, the end of which is brought into contact with a work piece. Readout means, often using circumferential vernier scales or digital readout devices, are provided to measure the axial movement of the spindle. For example, the spindle may be threaded so that rotation of the spindle causes the spindle to advance axially with respect to a micrometer frame. In another embodiment, the spindle is moved axially but does not itself rotate. In the latter embodiment, rotation of an interiorly threaded barrel affixed to the frame may operate to advance axially an exteriorly threaded spindle, rotation of the barrel indicating the distance advanced axially by the spindle.
The spindle is slideably received within a bushing which in turn is attached to the frame (or the spindle at least is prevented from axial movement with respect to the frame). For example, the digital readout micrometer referred to above may employ a bushing which is fixed to the spindle against relative rotation but through which the spindle may axially slide. The bushing is maintained stationary axially with respect to the frame. The spindle may bear an encoder wheel, rotation of which is detected by a stationary encoder mounted to the frame, the encoder delivering electric signals representative of the rotation of the encoder wheel (and hence also representative of axial movement of the spindle) for digital readout. In a similar embodiment, the bushing may bear a gear having teeth mechanically meshing with teeth of a mechanical readout device. In each of these situations, it is necessary that the encoder wheel, or gear, remain stationary from an axial standpoint with respect to the frame of the micrometer, and it is further necessary that the encoder wheel or gear be rigidly coupled or fixed to the micrometer spindle as the latter is rotated to advance it axially with respect to the frame.
In the embodiment referred to above in which the spindle does not itself rotate with respect to the frame, means may be provided to fix the spindle to a frame-mounted bushing to prevent relative rotation between the spindle and bushing while yet permitting the spindle to slide axially relatively freely and smoothly through the bushing.
Each of the embodiments referred to above requires that the spindle be substantially tightly coupled to the bushing against relative motion or movement therebetween in one direction (e.g., rotation), but that free movement of the spindle within the bushing be permitted in another direction (e.g., axially of the spindle).
One prior effort to provide the needed structure employed a set screw extending radially through the bushing, the inward end of the screw tapering to a point with the conical surface of the screw point riding in a V-shaped groove provided axially in the outer surface of the spindle. The operation of micrometers of this type is highly dependent upon the uniformity of the groove formed in the spindle, since the groove and set screw must provide for essentially no freedom of rotational movement between the spindle and bushing. Even minor irregularities in the configuration of the groove, accordingly, cannot be tolerated. The close fit which is thus required between the inner end of the set screw and the groove, however, presents other problems. For example, continued use of the micrometer will cause the end of the screw to wear, and hence continued, precise adjustment of the screw is necessary. Further, and perhaps of greater importance, slight imperfectations in the spindle groove, or small particles of foreign matter in the groove, may cause the closely fitting set screw head to bind in the groove, thereby causing rotation and axial advancement of the spindle to be abruptly halted. Considerable rotational momentum is often developed as the spindle is rapidly advanced toward, for example, the surface of a work piece, and the abrupt stoppage of movement of the spindle may not only damage the set screw head, but may cause considerable damage to associated gears and readout devices which are mechanically coupled to the spindle.